The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
In conventional friction stir welding (FSW) and friction stir processing (FSP), a rotating cylindrical tool is first plunged into the material to be processed or welded. Typically the material is a metallic material, though it may be possible to apply this to plastics, glasses, ceramics, and combinations thereof. The rotating cylindrical tool is then advanced horizontally along the surface of the material. In the case of FSW, the rotating tool is advanced along a butt joint between two abutting pieces or sections of the material to be joined. The rotating motion of the cylindrical tool and the heat generated in the material literally stirs material from both sides of the joint together, thereby producing a solid-state weld between the two pieces of material. This produces a joint between the materials that has superior material properties. In the case of FSP, the stirring causes the refinement of grains in the material and eliminates the porosity and voids in castings. In both FSW and FSP operations, frictional heat is generated between the tool and the welded material, which softens the material. Softening of the material below its melting point enables it to flow around the advancing, rotating tool.
FSW is an attractive solid-state joining process that can be performed in air and used to make very long joints in aluminum sheets used for constructing large structures with complex shapes, such as aircraft wings and fuselages. FSW can also be used for joining oxide dispersion strengthened (ODS) ferritic-martensitic (FM) steels that may be required for advanced nuclear reactor applications in the future. ODS FM steels are preferred for such applications since they have been shown to have exceptional high-temperature strength, high-temperature creep resistance, and resistance to radiation-induced swelling and creep. Unfortunately, conventional welding processes that require melting within the weld zone enable the Y2O3 nanodispersoid to phase separate from the molten metal, literally floating out of the weld zone. These particles can also agglomerate. This can result in a weakened weld after solidification. FSW avoids, or at least significantly mitigates, such problems.
It will thus be appreciated that FSW provides a solid-state joining process which has significant benefits and advantages over conventional fusion welding processes. By avoiding the formation of any liquid phase during processing, ODS FM steels can be joined without the loss of oxide particles from the weld, which typically lead to a weakened joint under some of the extreme operating conditions of interest. This solid-state process also enables the repair of high strength steels underwater, and the joining of aluminum and titanium under atmospheric conditions.
In spite of the foregoing advantages of FSW, FSW still has challenges to overcome. For example, an insufficient weld temperature can produce long tunnel-like imperfections (tunnel defects) in the weld that are difficult to detect, even with non-destructive evaluation (NDE) techniques. Therefore, there is still a need to better control the temperature of the material in the weld to significantly reduce or eliminate such long tunnel-like imperfections that may result from an insufficiently high material temperature at the weld joint.
FSP of nickel-aluminum bronze (NAB), which is used for propeller castings, has been shown to substantially improve the grain structure of the alloy and to dramatically improve the passive film stability on this complicated multi-phase alloy. NAB consists of a matrix of solid-solution alpha (α) phase (kii, kiii and kiv) and a finer and more complicated structure dispersed within this matrix, consisting of iron-aluminum intermetallics, known as the kappa (kii, kiii and kiv) phase (Farmer et al., “Studies of Passive Films on Friction Stir Processed and Laser Peened NiAl Bronze, Corrosion Resistant Metal Allows III, 2011,” Department of Defense Corrosion Conference, Jul. 31-Aug. 5, 2011, Palm Springs, Calif., NACE Paper 20194, National Association of Corrosion Engineers, Houston, Tex., 2011).
Unfortunately, the stress and wear experienced by FSW and FSP tools can shorten their service life, cause contamination of the welded or processed material with material lost from the rotating tool, and increase the cost per linear foot of weld produced, or the cost per square foot of surface processed. Each tool is quite expensive, costing approximately $3,000-$10,000 (US). If such a conventional FSW tool fails after about 75 meters (246 feet) of welding, which is the approximate maximum distance before failure observed by the inventors, then the cost per linear foot for tool replacement would be approximately $12-$41 per linear foot. This cost does not account for any of the other costs associated with the process, such as the cost associated with down-time needed to replace the tool. At the present time, a more realistic estimate for the total cost of friction stir welding, or friction stir processing is approximately $100-$1000 per foot.
Clearly, anything that can be done which cost effectively enhances the longevity of a tool used in a FSW or FSP operation would be highly desirable and valuable. Furthermore, anything that can be done to ensure that the temperature of the material being welded or processed is sufficiently high so as to maintain that material in a softened state, thereby enabling more facile flow around the advancing rotating tool is highly desirable, and commercially valuable.
Previous efforts to enhance the longevity of the tool have centered around preheating the material to be welded. This could be done using by placement of a localized heat source in front of the tool on the tool path, such as a laser [Palm, “Laser supported friction stir welding method.” U.S. Pat. No. 6,793,118. September 2004] or a flat induction coil [West et al., “Microstructure and Mechanical Properties of Friction Stir Processed Grade 40 Grey Cast Alloy”, Friction Stir Welding and Processing VI, edited by R. Mishra, M. W. Mahoney, Y. Sato, Y. Hovanski, and R. Verma, The Minerals, Metals & Materials Society, Pub. Wiley and Sons, Hoboken, N.J., 2011], or an electric arc [Kou, S., and Cao, G. “Arc-enhanced friction stir welding.” U.S. Pat. No. 7,078,647. July 2006].
In addition to a potentially sizeable back-end for high power lasers, the laser heating method requires a focused beam which can create hot spots in the region in front of the tool on the tool path, thereby affecting the load on the tool and hence the quality of the weld.
The use of flat induction coils is limited to magnetic materials, as the coil sits near the surface and does not surround the work-piece, and so can only affect it via magnetic hysteresis (as opposed to eddy currents). Furthermore, the coil needs to be in near intimate contact with the surface of the work piece, something that is not always possible (e.g. complex geometries). This limitation also exists for arc-assisted FSW and FSP.